The Northgate Link Extension light-rail tunnel, currently under construction in Seattle, includes RFID sensors built into some concrete liners to detect strain and help monitor each liner's integrity during construction, and potentially in the future, when the tunnel is in use. L-7 Services, an engineering firm based in Golden, Colo., designed the concrete liners, which are being installed in a parallel pair of 3.5-mile-long tunnels that will accommodate Sound Transit's rail service between the University of Washington, in Seattle, and Northgate, north of the city. The sensors, readers and software to manage the collected read data were provided by another Colorado company, Phase IV Engineering.

The Sound Transit tunnel is a three-year project that began in 2013. The challenge that L-7 faced in its design work was to ensure that the concrete liners would not lose structural integrity when any of them were cut to accommodate passageways connecting the two tunnels. For safety purposes, the tunnels require 23 such passageways, spaced about 800 feet apart, so that if a problem occurs involving an underground train in one tunnel, passengers could leave by walking to the nearest passageway and crossing over to the other tunnel.

Phase IV's RFID strain sensors, shown here attached to steel rods, are being installed in pairs (the pen is shown for scale).

L-7 Services' principal engineer, Kurt Braun, sought a way to verify that the liners' integrity was not compromised when openings were cut for the passageways. Therefore, he spoke with the general contractor, Jay Dee Contractors, which agreed that L-7 should look into options for embedding strain sensors in the concrete.

Braun found that wired sensors are bulky and difficult to install, so he began investigating wireless sensors and contacted Phase IV Engineering, which sells RFID-based strain sensors for use in bridge infrastructure. Phase IV developed the battery-assisted passive (BAP) SensTag RFID strain sensor for the Northgate Link project, along with a mobile reader with a telescoping antenna to interrogate the tags embedded in the liners. The firm also provides software to manage the sensor data so that Jay Dee and L-7 can be alerted in the event that the strain level exceeds an acceptable threshold. Sound Transit could also use the sensors in the future to intermittently monitor the tunnels' strain over time.

The sensor devices, manufactured by Phase IV and measuring approximately 1.5 inches by 1.5 inches by 0.5 inch, are each encased in a hard plastic cover. They come with a built-in strain sensor, a passive ultrahigh-frequency (UHF) RFID tag and a battery so that the tag can automatically measure the strain level without being interrogated by an RFID reader, and then save that data in the tag's memory. The battery has a lifetime of about 10 years, according to Scott Dalgleish, Phase IV Engineering's CEO.

"We're proud that the battery can last 10 years," Dalgleish adds, though he adds that "Tunnels have a hundred-year life." As such, Phase IV wanted the tags to be of value potentially for the duration of the tunnel's lifetime.

Therefore, the tags are designed to operate whether the battery is working or not, so that the sensor will continue functioning long after the battery has expired. Initially, the device serves as a data-logger, storing periodic measurements. When an individual brings a reader to interrogate the tag, it captures not only the tag ID number and current sensor reading, but also the history of other measurements taken in the past. The sensors are being programmed to carry out three readings daily, though the frequency of measurements can be changed at any time by means of a reader.

Once the battery dies, the tag can still operate in passive mode. Thus, in order to read the tag after its battery has stopped functioning, a user would need to bring the reader antenna within about 12 inches of the tagged concrete's surface. The tag would then use the power of the device's RF signal to take a strain measurement and send that to the reader. From that point on, it would no longer provide data-logging capability.

However, Braun says, his initial interest in the technology was to ensure that the passages cut into the tunnel did not compromise the liners.